A surprising number of big cites are located by river rapids

Once upon a time in America, rivers played a major role in transporting good, and many portage sites, where boats would unload to get around rapids, grew into large settlements.

Centuries later, even as portage sites have become insignificant, cities located near them have remained particularly large.

Economists Hoyt Bleakley and Jeffrey Lin call this evidence of path dependence: settlements that have an early advantage tend to hold onto their head start. They wrote about the phenomenon in a 2012 paper (referenced in a new paper on satellite-based economics).

Here's a pretty map of the US, showing rivers, the fall line (i.e., the last major rapids before the sea), and night lights as a measure of economic activity. Cities on rivers near the fall line tend to be bigger.